Mixtures of solid particles can separate or segregate during handling. The non-uniformity of the mixture can result in quality control problems, such as the waste of raw materials, lost production, and increased maintenance and capital costs required to retrofit existing facilities where unwanted segregation of solid particle flows is occurring. Segregation problems can occur with a number of different types of solid particle mixtures, including larger particles, such as coal or rocks, to smaller particles, such as powders, including pharmaceutical powders.
Segregation can occur in a number of different ways, based primarily on various physical properties of the mixture and environmental or handling conditions. Sifting is a prevalent form of segregation. Sifting can be defined as the movement of smaller particles through a mixture of larger particles. This can occur during formation of a pile, as smaller particles percolate into the pile, while coarse particles slide or roll to the perimeter of the pile. In order for sifting segregation to occur, several conditions are required. There must be a difference in particle size, for example, ratios as small as 1.3:1 can induce sifting segregation. Sifting is generally most pronounced when the mean particle diameter is greater than 100 microns. The mixture must be sufficiently free flowing to allow interparticle motion. Finally, there must be movement of the particles relative to one another or portions of the flow within the mixture.
Bulk storage containers, such as hoppers, silos, bunkers and bins, are conventionally used for the storage of quantities of loose particulate solids, including particulate solid mixtures. For the purposes of the present application, the term “hopper” will be used to cover all such differing forms of storage containers for particulate material, where the material fills or partially fills the container and moves during the discharge process to an outlet situated in the lower regions of the container. If all of the material is in motion during discharge, this is referred to as mass flow of the material.
Bulk solids are generally comprised of particles of different sizes. It is commonly desirable to maintain a uniform concentration of each size throughout the mixture during industrial processing, storage, and packaging. However, segregation of the particles by size frequently occurs during processing steps such as the filling or discharge of a hopper. Such actions can lead to segregation by sifting. Accordingly, different regions within a mixture of particulate solids within a hopper can have different proportions of fine and coarse particles. Thus, uniformity of the mixture is lost.
Conventional simulation test practices have been devised to obtain useful data on the tendency of a particular body of solids to undergo significant segregation when transferred under industrial conditions. These practices simulate the industrial conditions using a quantity of the solids involving filling and discharge of a test chamber. As part of the testing procedure, multiple samples are taken during discharge of the test chamber and subjected to separate assays and analyses of the properties of interest. These results for a new material may be compared with the results for other solids having known segregation properties when similarly tested in the same apparatus and with the same filling test procedure. This comparison provides an indication of potential of the new material to segregate by sifting in a given industrial application.
The present invention provides an improved method for segregation testing. The resulting samples may be evaluated using presently existing or future techniques.
A standard practice for measuring sifting segregation tendencies of bulk solids is described by ASTM International under Designation D6940-03. The bulk solid is filled into a test chamber, creating a pile. The pile formation may cause sifting segregation of the material. For example, segregation may cause the finer particles to concentrate under the central fill point, while the coarser particles concentrate at the perimeter of the pile. The segregated material is then discharged from the test chamber in a funnel flow pattern, intended to recover zones of segregated material (e.g. center first, perimeter last) in a known sequence. Samples are collected from the discharge stream. The samples are then analyzed for differences that are relevant to the application, such as particle size or assay.
In practice, the existing ASTM standard practice and its variations have a number of drawbacks. For example, the existing standard test has proven ineffective for certain materials that do not flow easily. The funnel flow pattern tends to be unreliable with cohesive materials, resulting in arching and ratholing, which can cease flow of material.
The ASTM standard practice uses a liter of material. This is often more material than the material actually available during early stages of development of a particulate solid material, such as for a pharmaceutical formulation. Attempts to reduce the size of the current test chamber, while employing the same geometric ratios described in the ASTM method, are ineffective for cohesive materials, since arching and rat-holing are more likely to occur, and the segregation is less pronounced.
The conventional ASTM standard test procedure typically yields individual samples of substantial volume that, although suitable for certain types of analysis, are often too large for some other common analytical methods. As a result, time consuming sub-sampling, sample splitting or riffling procedures are needed to obtain representative smaller quantities for analysis. Sub-sampling can also lead to errors and material loss.